FIG. 8 shows a system diagram of a prior art DS-PSK (Direct Sequence--Phase Shift Keying) modulation apparatus and FIG. 9 shows a system diagram of a prior art DS-PSK demodulation apparatus, which are disclosed in page 18.
In FIG. 8, reference numeral 1 designates a data input terminal. Reference numeral 2 designates a psuedo noise (hereinafter referred to as "PN") code generator. Reference numeral 3 designates a mixer which spreads the spectrum by multiplying the data by the PN code. Reference numeral 4 designates a PSK modulator which modulates the output of the mixer 3. Reference numeral 5 designates an output terminal for the spread spectrum signal.
In FIG. 9, reference numeral 6 designates an input terminal for received spread spectrum signal. Reference numeral 7 designates a PN code generator which generates a PN code which is synchronized with the output of PN code generator 2 at the transmitter's side. Reference numeral 8 designates a mixer which demodulates the received spread spectrum signal to the narrow band PSK signals multiplying by the PN code which is generated by the PN code generator 7 at the receiver's side. Reference numeral 9 designates a PSK demodulator. Reference numeral 10 designates an output terminal for reproduced data.
FIG. 11 shows waveforms of the respective portions of DS-PSK modulator and demodulator. In FIG. 11, reference numeral 11 designates an input data. Reference numeral 12 designates a PN code for transmission. Reference numeral 13 designates a mixer output for transmission. Reference numeral 14 designates a PSK modulation output which is also input to the PSK demodulator. Reference numeral 16 designates a mixer output for receiving. Reference numeral 17 designates a reproduced data output.
The operation of this device will be described with reference to FIG. 11.
The input data 11 and the PN code 12 are spread by the mixer 3, and a mixer output 13 is obtained. This mixer output 13 is modulated by the PSK modulator 4, to result in a PSK modulated waveform 14.
This PSK modulated waveform 14 is input to the DS-GMSK demodulator and is demodulated to the narrow band PSK signals by multiplying it by the receiver's PN code 15 at the receiver's mixer 8, to result in a mixer output 16. This mixer output 16 is demodulated by the PSK demodulator 9, to result in a reproduced data 17.
Next, the DS-MSK and DS-GMSK modulator and demodulator which are easily assumed from the above-described DS-PSK system will be described.
FIG. 12 show a DS-MSK modulator or a DS-GMSK modulator modulated by PSK as data modulation (hereinafter referred to as DS-MSK/GMSK-PSK) and FIG. 13 shows that modulated by FSK as data modulation (hereinafter referred to as DS-MSK/GMSK-FSK) respectively, which are easily assumed from the DS-PSK modulator and demodulator, and FIG. 14 shows a DS-MSK or DS-GMSK demodulator. In these figures, reference numeral 18 designates a MSK or GMSK modulator. Reference numeral 18a designates a VCXO (Voltage Controlled Crystal Oscillator). Reference numerals 1 to 10 designate the same elements as those shown in FIG. 8.
FIGS. 15 and 16 show waveforms of respective portions of DS-MSK/GMSK-PSK modulator and DS-MSK/GMSK-FSK modulator of FIGS. 12 and 13, respectively.
The device will operate as follows. In FIG. 12, the output 33 of MSK or GMSK modulator 18 which is modulated by the PN code 32 and the input data 31 are multiplied with the output 33 of modulator 18 at the mixer 3. On the other hand, in FIG. 13, VCXO 18a is used as an oscillator of MSK or GMSK modulator 18, and the VCXO output 36 is spread by the PN code 37 at the mixer 18b, to result in a DS-MSK/GMSK FSK (Frequency Shift Keying) modulated waveform 38.
In FIG. 14, the demodulation of spread spectrum signals is conducted to the dispersion signal by mixing an MSK dispersion signal or GMSK received signal by mixing an MSK or GMSK signal modulated by the same sequence as the PN code generator in the transmitter's side. Modulation apparatus of FIG. 12, a PSK signal is obtained and for the output signals of the modulation apparatus of FIG. 13, an FSK reproduced signal is obtained.
The prior art DS-MSK and DS-GMSK modulation apparatus are constituted as described above, and in the system of FIG. 12, a steep phase shift arises at the changing point of data. This deteriorates the characteristics of MSK and GMSK signals, the systems having less extra band spectrum and less extra band noise. Furthermore, in the system of FIG. 13, since a frequency modulation is executed to the carrier, the extraction of carrier from the input signal is difficult. Further, VCXOs are required in the MSK or GMSK modulators.